Office automation is becoming increasingly prevalent, and use of color documents is growing. In the past, office equipment was mainly used for taking copies of documents consisting only of text. Now, documents including graphics such as graphs are prepared in personal computers, printed out from printers, and large amounts of copies are taken to produce presentation materials, for example. Images output from printers include solid images, line images, and halftone images. Thus, demands for image quality are changing, and high reliability is increasingly demanded.
Electrophotographic methods such as electrostatic recording and electrostatic printing include a developing process for developing an electrostatic image on an image carrier such as a photoconductor by applying a developer to the photoconductor, a transfer process for transferring the developed image from the photoconductor to a transfer medium such as paper, and a fixing process for fixing the image onto the paper. There are two types of developers for developing the electrostatic image formed on the photoconductor, i.e., a two-component developer including carriers and toner, and a single-component developer, which does not require carriers, including only magnetic toner or nonmagnetic toner. The two-component developer has the following disadvantages: the developer deteriorates as toner particles stick to the surfaces of the carriers; and a mixture of the toner and the carriers needs to be maintained at a certain ration, so that toner density in the developer does not decrease as the toner is consumed. Accordingly, a large-sized developing device is needed to realize such a configuration. On the other hand, the single-component developer is advantageous in that the developing device can be made compact, and that the developer can be used under any temperature or humidity conditions. Accordingly, the single-component developer is becoming a mainstream.
There are two types of single-component developers, i.e., a magnetic single-component developer including magnetic toner, and a nonmagnetic single-component developer including nonmagnetic toner. In a magnetic single-component developing method employing the magnetic single-component developer, a developing sleeve with a magnetic field generator such as a magnet provided inside holds the magnetic toner including magnetic substances such as magnetite, and a layer thickness restricting member reduces the thickness of the toner for the developing process. The magnetic single-component developer is widely used in compact printers. In a nonmagnetic single-component developing method employing the nonmagnetic single-component developer, the toner does not have a magnetic force, and therefore, a toner supplying roller is pressed against a developing sleeve to supply the toner to the developing sleeve, and the developing sleeve holds the toner by static electricity. A layer thickness restricting member reduces the thickness of the toner for the developing process. Because the toner does not include chromatic magnetic substances, the nonmagnetic single-component developer is useful for producing color images, and because the developing sleeve does not include a magnet, a light-weight, low-cost developing device can be realized. Accordingly, the nonmagnetic single-component developer is widely used in compact, full-color printers.
However, the single-component developing method has many problems to be solved. In the two-component developing method, the carriers are used to electrically charge and convey the toner. The toner and the carriers are sufficiently mixed and stirred together, and then conveyed to the developing sleeve for the developing process. Therefore, the two-component developer can be steadily charged and conveyed over a long time, and can be used in a high-speed developing device. On the other hand, the single-component developing method does not employ carriers that can steadily charge and convey the toner, and therefore, failures occur in the charging and conveying operations when the developing device is used over a long time or at a high speed.
Particularly in the nonmagnetic single-component developing method, the toner contacts friction-charged members such as the developing sleeve or the layer thickness restricting member only for a very short time. Therefore, there is a higher chance of creating low charged toner or reversely charged toner than in the two-component developing method. Furthermore, the layer thickness of toner on a toner conveying member, which conveys the toner to the image carrier, needs to be as thin as possible. Accordingly, the toner conveying member receives a force from the layer thickness restricting member, which pushes outer additives on the surface of the toner particles inside the toner particles. This significantly deteriorates the chargeability and the flowability of the toner.
To solve the above problems, technologies are disclosed in, for example, Japanese Patent Application Laid-Open No. H08-122559 and Japanese Patent Application Laid-Open No. 2005-062215.